PURPOSE: To prevent a lug from being formed outside of a hull by constituting said cover device so that the cover proper can be lifted or lowered and stored in a trunk body to open a duct.

CONSTITUTION: In a cover device, a cover proper 7 is opened or closed by an operation of a drive mechanism 8. Under a normal condition, the cover proper 7 closes a duct opening 101 nearly at the same surface as the hull shape without forming any lug outside of the hull. When the drive mechanism 8 is operated at this condition, the cover proper 7 is lifted while being guided by a rail body 9 and stored in a trunk body 6 to open a duct. At this time, the cover proper 7 is only drawn into the trunk body 6 above, thus the duct can be opened with no resistance.